Conventionally, developers generally used in electrophotography can have desired flowability and charging characteristics by attaching external additives on the surfaces of colored resin particles. As external additives, inorganic particles and organic particles are generally used. As such external additives, metal oxide particles, resin particles and these particles subjected to a surface treatment have been widely used. Among them, particularly, particles of metal oxides such as silica, titania, alumina and zinc oxide, particles of fatty acid metal salts, and these particles subjected to a hydrophobicity-imparting treatment are often used. They are also generally used in combination of two or more kinds.
For example, in Patent Literature 1, an electrostatic image developing toner made of toner particles and zinc oxide fine particles is disclosed, wherein the toner particles contain particles essentially made of a thermoplastic resin-based binder and a pigment, and the zinc oxide fine particles are attached on the surface of the toner particles and covered with a modified silicone oil that contains at least one organic group selected from an amino group and an epoxy group. It is also disclosed that the toner gives an image with less fog and is excellent in durability. In Patent Literature 2, a negative charge type toner obtained by covering spherical polyester resin particles containing colorant particles with a plurality of hydrophobized external additives is disclosed, wherein at least negative charge type silica particles, rodlike polyhedral hexagonal zinc oxide particles and positive charge type silica particles are used as the external additives. It is also disclosed that the toner is excellent in charge stability, ensures neither toner leakage nor toner scattering, and gives a printed image free of unevenness.
In Patent Literature 3, a positively chargeable toner is disclosed, which contains toner base particles surface-treated with an external additive that contains zinc oxide fine particles subjected to a positive charging property-imparting treatment and a silicone oil treatment, in which the amount of an agent used in the positive charging property-imparting treatment and the amount of the silicone oil used in the silicone oil treatment are at a specific ratio. It is also disclosed that the positively chargeable toner causes no decrease in charge amount, even in long-term use, and gives an image that hardly causes toner scattering or fog.
However, in different environments, the toners disclosed in these patent literatures are insufficient to inhibit fog, and sometimes they are difficult to maintain a toner conveyance amount that is close to the beginning of printing even during continuous printing, with satisfying a latest demand for high speed printing and maintaining low-temperature fixability.